you can convert most anything if you know the formats and write the utility, or find someone that made such utility already.

Many game titles get published using 3DGS but you never see them associated to 3DGS or even listed on the 3DGS site. they arn't required to.

No your not "STUCK" to one engine. You will find many developers use more than one. which one they use is depending on the particular projects needs